Data readers

ABSTRACT

A data reader arranged to produce a signal on reading a data holding medium ( 4 ), said data reader comprising processing circuitry ( 8 ) arranged to process said signal, said processing circuitry including a filter ( 14 ) having a variable cut-off frequency (F c ), a velocity signal generator ( 23 ) arranged to produce a signal ( 21 ) corresponding to the velocity of the data holding medium ( 4 ), and a processor ( 22 ), said processor ( 22 ) being arranged to read the velocity signal ( 21 ) and set the cut-off frequency (F c ) of the filter ( 14 ). The data reader is generally arranged to be incorporated into a tape drive ( 2 ) arranged to be used as a computer data storage device.

This invention relates to an improved data reader and an improved methodof reading data. It is particularly applicable to data storage devices,but may have wider applicability.

Early magnetic tape storage devices moved the magnetic tape past readheads at a fixed velocity. A low pass filter is provided within thedecoding circuitry that removes unwanted high frequency noise. Becausethe tape passes the read head at a fixed velocity a signal produced byreading data read from the tape has a known maximum frequency.Therefore, the cut off frequency of the low pass filter can be set toensure all of the signal passes the filter.

It is now known to produce magnetic tape storage devices in which thevelocity of the tape past the read head is varied. Varying the velocityin this manner helps to ensure that the rate of data transfer to andfrom the tape can match the rate of data transfer to the storage device.This matching of data rates helps to prevent unnecessary stopping of thestorage device. Stopping causes wear to the drive mechanisms andtherefore, wear can be reduced if the drive can be slowed rather thanstopped.

However, by altering the velocity at which the tape passes the readhead, the frequency of the signal produced on reading data from the tapeis altered. Therefore, it is desirable that the cut-off frequency of thelow pass filter is altered accordingly. It is undesirable to have thecut-off frequency set too far above the maximum frequency of the signalsince noise will not be effectively removed. Further, if the cut-offfrequency is set too low then a portion of the signal will be lost.Generally, as the tape speed increases, data rate increases and it isnecessary to increase the level of the cut-off frequency in order thatthe higher frequency data are not filtered or attenuated.

Prior solutions to this problem are known and an example is shown inFIG. 1. In this example a Phase Locked Loop (PLL) is used to look ontothe clock derived from the tape velocity. Control currents used in thePLL to adjust internal analogue parameters such that the PLL locks tothe clock frequency are also fed to the filter. These control currentscause the cut off frequency of the low pass filter to be set at thecorrect position for the particular clock frequency.

This technique relies on the matching of components in the PLL and thefilter. This can be difficult to achieve over fabrication processcorners and for the whole frequency range. These difficulties can resultin a poor yield in the fabrication process.

It is an object of the present invention to provide a data storagedevice that is easier to fabricate than the prior art.

According to a first aspect of the invention there is provided a datareader arranged to produce a signal on reading a data holding medium,said data reader comprising processing circuitry arranged to processsaid signal, said processing circuitry including a filter having avariable cut-off frequency, a velocity signal generator arranged toproduce a signal corresponding to the velocity of the data holdingmedium, and a processor, said processor being arranged to read thevelocity signal and vary the out-off frequency of the filter insubstantially linear relation to variations in the velocity signal onthe basis of data generated during a calibration process of the datareader.

An advantage of such a data reader is that it is easier to fabricatethan prior art data readers.

Preferably, the filter is a low pass filter.

The velocity signal generator may be provided by a read head arranged toproduce the signal on reading a data holding medium. The velocity signalis preferably a clock signal produced by the read head. Conveniently thefrequency of the clock signal corresponds to the velocity of the tapepast the read head. It is advantageous to have a clock signal as thevelocity signal since this is readily read by the processor.Alternatively, an analogue signal may be produced, but it is likely thatsuch an analogue signal would need digitising before being able to beread by the processor.

Preferably, the read head is arranged to read markers on thedata-holding medium in order to produce the velocity signal. Such anarrangement provides a simple way of allowing the velocity signal to begenerated.

The processor may be arranged to determine the frequency of the velocitysignal in order to vary the cut-off frequency of the filter.

In any case, the processor may be arranged to vary the appropriatecut-off frequency of the filter by referring to a look up table, thevalues of which look up table may have been generated during thecalibration process. Use of a look up table in this manner provides asimple, yet effective system for controlling the cut-off frequency.

Alternatively, the processor may set the cut-off frequency by applying arepresentation of the velocity signal to a function such as a polynomialfunction weighted to generate an appropriate output to control thecut-off frequency of the low pass filter.

Conveniently, the processor includes an output register arranged suchthat the register's content controls cut-off frequency of the filter.Such an output register is convenient since is provides a simpletechnique to output the desired out-off frequency.

Preferably a Digital to Analogue Converter (DAC) is provided within theprocessing circuitry, arranged to produce an analogue signal to controlthe cut-off frequency of the filter.

Preferably, the DAC is arranged to have input thereto the value that iscontained in the output register of the processor. Such an arrangementprovides a convenient structure for controlling the filter.

The processor may be arranged to perform a self-test routine in whichthe values that are contained in the look up table are adjusted. Such anarrangement is convenient because it allows minor discrepancies in thevalues that are contained in the look up table to be corrected.Therefore, the control of the cut-off frequency of the filter should bemore accurate.

The filter may be arranged to have an increased gain in the region ofthe cut-off. This is advantageous because it provides the necessaryequalisation to achieve the desired signal characteristics.

Preferably, the reader is arranged to cause the velocity of thedata-holding medium to be varied over any velocity within apredetermined range. In one embodiment the maximum velocity is limitedto roughly three times the minimum velocity. Other ratios of maximum tominimum are equally possible: For instance roughly any of the followingmay be suitable: 2 to 1, 4 to 1, 5 to 1, 6 to 1, 8 to 1, or 10 to 1, orindeed any value in between these ranges.

In the preferred embodiment the maximum tape velocity is roughly 4.1m/s. However, the maximum tape velocity may be roughly any of thefollowing values: 1 m/s, 2 m/s, 3 m/s, 5 m/s, 6 m/s, 7 m/s, 8 m/s, 9m/s.

Alternatively, or additionally, the reader is arranged to cause thevelocity of the data-holding medium to be varied over a number ofpredetermined velocities within a predetermined range.

According to a second aspect of the invention there is provided a datastorage device including a data reader according to the first aspect ofthe invention.

Advantageously, the storage device is provided with a buffer arranged toreceive data sent to the device (and/or buffer data sent from thedevice). The data reader may be arranged to cause the velocity of thedata-holding medium to be varied according to the amount of data presentin the buffer.

In one embodiment the data storage device is arranged to receivemagnetic tapes wherein the magnetic tape provides the data-holdingmedium. However, the device may be arranged to read data from a harddisk wherein the disk platter is the data-holding medium. The storagedevice may be arranged to read data from other forms of data-holdingmedium.

The storage device may be any of the following types of tape drive andfor example may be any of the following: DAT (Digital Audio Tape), DLT(Digital Linear Tape), DDS (digital Data Storage), or LTO (Linear TapeOpen), or any other type.

The storage device may be arranged to communicate with other devices viaany form of bus. The bus may be SCSI, Firewire, USB, Fibrechannel, etc.

According to a third aspect of the invention there is provided a methodof reading data from a data-holding medium to produce an output signal,the method comprising determining the velocity of the data holdingmedium and varying the cut-off frequency of a filter in substantiallylinear relation to variations in the velocity of the data holding mediumon the basis of data generated during a calibration process of the datareader An advantage of such a method is that it is easier to performthan prior art methods of reading data from a data-holding medium.

Conveniently, the processor consults a look up table to vary the cut-offfrequency. Such a method provides a convenient way of determining thecut-off frequency.

Alternatively, other methods of determining the cut-off frequency may beutilised by the processor. For instance, the processor may apply apredetermined function to a representation of the velocity of thedata-holding medium to determine the cut-off frequency.

The method may comprise performing a self calibration routine in whichthe processor adjusts values contained in the look up table/adjusts thepredetermined function to help ensure that the cut-off frequency of thefilter is correctly controlled relative to the velocity of the dataholding medium. This helps to ensure that data can be accurately readfrom the data-holding medium.

Conveniently, the filter is arranged to provide gain to a signal fedthereto in a region of the cut-off frequency. This helps to equalise thesignal to the desired signal characteristics.

Preferably, the processor utilises a clock signal generated by a readhead to determine the velocity of the data-holding medium. Read heads ofdata storage devices generally produce such a clock signal. Therefore,utilising this signal is a convenient way of determining the velocity.

There now follows by way of example only a detailed description of theinvention with reference to the accompanying drawings of which:

FIG. 1 shows a prior art arrangement for processing a signal read from amagnetic tape;

FIG. 2 shows the main components of a storage device;

FIG. 3 shows a schematic view of an arrangement for initially processinga signal produced on reading a magnetic tape;

FIG. 4 schematically shows the cut off frequency of a low pass filter inrelation to the signal produced on reading the magnetic tape;

FIG. 5 shows schematically the components for initially processing asignal produced on reading the magnetic tape according to the presentinvention;

FIG. 6 is a flow chart outlining how the components shown in FIG. 5 arecontrolled;

FIG. 7 schematically shows the layout of a magnetic tape capable ofbeing read by the present invention;

FIG. 8 shows the gain for a low pass filter according to one embodimentof the invention.

This invention will be described in relation to a magnetic tape datastorage device, although it may have wider applicability. The basiccomponents of a magnetic tape storage device 2 are shown in FIG. 2. Adata-holding medium, in this case a magnetic tape 4, is arranged to beread by a read head 6, which produces a signal that is fed to processingcircuitry 8. The processing circuitry generates an output signal that isfed to an output port 10.

FIG. 3 shows the read head 6 and some of the processing circuitry inmore detail. The processing circuitry is arranged to pass the signalgenerated by the read head 2 on reading the tape 4 to a variable gainamplifier 12, which amplifies this signal. This amplified signal is fedto a low pass filter 14 arranged to remove unwanted noise above anappropriate cut-off frequency.

FIG. 4 shows an example of the relationship between the envelope 16 forthe frequencies contained in the amplified signal compared to thecut-off frequency f_(c) of the low pass filter 14. The cut-off frequencyf_(c) should be such that all of the frequencies with the envelope 16pass the filter without being attenuated.

However, in a tape storage device 2 in which the velocity of the tape 4is varied to vary the data rate the frequencies contained in theenvelope 16 also vary. Therefore, to ensure that the cut-off frequencyf_(c) is not too high or too low (ie let through too much noise, orremove some of the wanted frequencies respectively) the processingcircuitry 8 is arranged to vary it.

FIG. 5 shows the blocks used to control the value of the cut-offfrequency. The read head 6 produces a signal on reading the tape 4. Adecoding processor 23 generates a clock frequency that is proportionalto the velocity of the tape 4. On the tape 4 a series of markings 18 areprovided in addition to the tracks of data 20 (best seen in FIG. 7). Themarkings 18 are read by the read head 4, which produces a clock signal21 corresponding to the rate at which the markings pass the read head 6.

This clock signal 21 is fed to a processor 22, which determines thefrequency of the clock signal 21 and consequently determines thevelocity of the tape 4 passing the read head 6. In the presentembodiment, the processor 22 has associated therewith a look up table24, which contains a list of register values for various velocities oftape 4. The look up table 24 could of course be provided with memoryexternal to the processor 22 such as E²PROM, or other non-volatilememory or possibly within dedicated memory provided within the processor22. The values contained in the look-up table 24 are determined at thetime of device manufacture and are specific to each data reader.

The processor 22 includes an output register 26, which is arranged toreceive the value which, when applied to the low pass filter, causes thecut-off frequency f_(c) of the filter 14 to be set to the appropriatevalue. A Digital to Analogue Converter (DAC) 28 is provided and arrangedto convert the digital value, placed by the processor, into the outputregister 26 into an analogue signal. The analogue signal 30 produced bythe DAC 28 is fed to the filter 14 such that the cut-off frequency f_(c)is varied appropriately. The analogue signal is presented as a voltageor current respectively depending on whether the filter cut-offfrequency is voltage or current controlled.

In some embodiments the filter 14 is arranged such that there is anamount of gain in the region of the cut-off frequency f_(c). An exampleof this is shown in FIG. 8.

In use, the tape 4 is inserted into the storage device 2 such that theread head 6 can read it. As the tape 4 passes the read head 6 themarkings 18 on the tape are read 32 and used to produce the clock signal21. The velocity of tape 4 is varied to alter the rate at which data ismoved to/from the tape 4 to/from a device connected to the storagedevice 2 via the port 10.

As the velocity of the tape 4 varies the frequency of the clock signal21 varies. Therefore, as the clock signal 21 is input to the processor22 (block 34) the processor 22 can determine the velocity of the tape 4by determining the frequency of the clock signal 21. Once the frequencyhas been determined the processor looks up in the look up table 24(block 36) the output value to generate the required filter cut offfrequency.

The output value determined by looking in the look up table 24 is placedinto the output register 26, and converted to an analogue signal 30 bythe DAC 28. This analogue signal 30 is input to the filter 14 to controlthe cut-off frequency f_(c) (block 38).

According to the present embodiment, the relationship between the tapevelocity and the cut-off frequency is substantially linear throughoutthe required range. The values in the look up table are set during adrive calibration process at the time of manufacture, wherebynon-linearities are factored-out by setting appropriate values in thelook up table. Because the calibration process maps the tape velocity tothe required value of f_(c), compensation can be made for any of thesenon-linearities so that velocity versus f_(c) is linear. One example ofa calibration process would be to increment the tape velocity in alinear fashion and, for each increment, vary the respective value in thelook up table in order to obtain the required cut-off frequency. Otherappropriate calibration procedures would be apparent to the skilledperson.

If the relationship is not linear then the drop out level in data readby the reader may increase significantly (due to increased noise becausethe cut-off frequency is set too high, or to loss of signal because thecut-off frequency is set too low). Compensating for non-linearity allowsthe cut-off frequency to be set at the correct level.

The skilled person will appreciate that the term processor is envisagedto cover a range of different types of circuit: micro-controllers,microprocessors, ASIC's, Programmable Logic Arrays (PLA), hardwiredcircuitry of discrete components, etc.

In alternative embodiments, the look up table may be replaced by afunction for generating a processor output value, which causes the tapevelocity to have a linear relationship with the cut-off frequency. Ineffect, the function would compensate for any intrinsic non-linearity.One function would be a polynomial of the form a+bx+cx²+dx³+ . . .+nX^(m), where x represents the tape velocity signal value and thecoefficients a, b, c, d, . . . , n are set at appropriate values tocompensate for any non-linear relationships between the tape velocityand the resultant f_(c). A similar calibration procedure as the onedescribed above to calibrate the look up table could be used tocalibrate the polynomial. Instead of varying the look up table values,however, calibration of the polynomial would require varying thecoefficients a, b, c, d in order to arrive at the correct output.

1. A data reader arranged to produce a signal on reading a data holdingmedium, said data reader comprising processing circuitry arranged toprocess said signal, said processing circuitry including a filter havinga variable cut-off frequency, a velocity signal generator arranged toproduce a signal corresponding to the velocity of the data holdingmedium, and a processor, said processor being arranged to read thevelocity signal and vary the cut-off frequency of the filter insubstantially linear relation to variations in the velocity signal onthe basis of data generated during a calibration process of only theparticular data reader.
 2. A data reader according to claim 1 whereinthe velocity signal generator comprises a read head arranged to producesaid velocity signal on reading a data holding medium.
 3. A data readeraccording to claim 2 wherein said read head is arranged to read markerson said data-holding medium in order to produce said velocity signal. 4.A data reader according to claim 1 wherein said processor is arranged todetermine the frequency of said velocity signal in order to vary saidcut-off frequency of said filter.
 5. A data reader according to claim 1wherein said processor is arranged to vary said cut-off frequency ofsaid filter by referring to a look up table, the values of which look uptable were generated during the calibration process.
 6. A data readeraccording to claim 1 wherein said processor includes an output registerarranged such that the contents of said register controls said filtercut-off frequency.
 7. A data reader according to claim 6 wherein aDigital to Analogue Converter (DAC) is provided within said processingcircuitry and is arranged to produce an analogue signal to control saidfilter cut-off frequency.
 8. A data reader according to claim 7 whereinsaid DAC is arranged to have input thereto the value that is containedin said output register of said processor.
 9. A data reader according toclaim 8 wherein said filter is arranged to have an increased gain in theregion of said cut-off.
 10. A data reader according to claim 1 arrangedto cause the velocity of said data-holding medium to be varied over anyvelocity within a predetermined range.
 11. A data storage deviceincorporating a data reader according to claim
 1. 12. A data storagedevice according to claim 11 wherein a buffer is provided and arrangedto receive data sent to said device.
 13. A data storage device accordingto claim 12 wherein said data reader is arranged to cause the velocityof the data-holding medium to be varied according to the amount of datapresent in the buffer.
 14. A data storage device according to claim 13that is arranged to receive magnetic tapes wherein said magnetic tapeprovides said data-holding medium.
 15. A method of reading data from adata-holding medium to produce an output signal, the method comprisingdetermining the velocity of the data holding medium and varying thecut-off frequency of a filter in substantially linear relation tovariations in the velocity of the data holding medium on the basis ofdata generated during a calibration process of only the particular datareader.
 16. A method according to claim 15 including consulting a lookup table to vary the cut-off frequency.
 17. A method according to claim16 in which the values in the look up table are set by the calibrationprocess.
 18. A method according to 15 in which the filter is arranged toprovide gain to a signal fed thereto in a region of said appropriatecut-off frequency.
 19. A method according to claim 15 in which theprocessor utilises a clock signal generated by a read head to determinethe velocity of the data-holding medium.